This invention relates to antennas to receive signals from Global Positioning System (GPS) satellites and, more specifically to antenna systems arranged for reception for differential GPS applications.
Antenna systems providing a circular polarization characteristic in all directions horizontally and upward from the horizon, with a sharp cut-off characteristic below the horizon are described in U.S. Pat. No. 5,534,882, issued to A. R. Lopez on Jul. 9, 1996. Antennas having such characteristics are particularly suited to reception of signals from GPS satellites.
As described in that patent, application of the GPS for aircraft precision approach and landing guidance is subject to various local and other errors limiting accuracy. Implementation of Differential GPS (DGPS) can provide local corrections to improve accuracy at one or more airports in a localized geographical area. A DGPS ground installation provides corrections for errors, such as ionospheric, tropospheric and satellite clock and ephemeris errors, effective for local use. The ground station may use one or more GPS reception antennas having suitable antenna pattern characteristics. Of particular significance is the desirability of antennas having the characteristic of a unitary phase center of accurately determined position, to permit precision determinations of phase of received signals and avoid introduction of phase discrepancies. Antenna systems having the desired characteristics are described and illustrated in U.S. Pat. No. 5,534,882, which is hereby incorporated herein by reference.
For such applications, antennas utilizing a stack of individually-excited progressive-phase-omnidirectional elements are described in U.S. Pat. No. 6,201,510, issued to A. R. Lopez, R. J. Kumpfbeck and E. M. Newman on Mar. 13, 2001. Elements as described therein include self-contained four-dipole elements which are employed in stacked configuration to provide omnidirectional coverage from the zenith (90° elevation) to the horizon (0°) or from a high elevation angle to the horizon, with a sharp pattern cut off below the horizon. U.S. Pat. No. 6,201,510 is hereby incorporated herein by reference.
In some applications, it may be desirable to employ a set of two antennas, each providing omnidirectional coverage (in azimuth) and the antennas providing complementary coverage in elevation. For example, an antenna of the type described in U.S. Pat. No. 6,201,510 may be designed to provide omnidirectional coverage from the horizon to a specified elevation angle. If available, a second high-angle omnidirectional antenna of appropriate design and performance could be used to provide complementary elevation coverage from that elevation angle to the zenith. Used together, such antennas would provide horizon to zenith coverage for omnidirectional reception of GPS signals for DGPS applications. Available antennas for high elevation angle coverage have generally been subject to limitations in areas such as performance, size, cost, reliability or compatibility for integration into a single dual-array antenna.
Objects of the present invention are to provide new and improved antennas, including antennas usable for DGPS applications and having one or more of the following characteristics and advantages:                dual-array two-port configuration;        omnidirectional azimuth coverage with elevation coverage from horizon to zenith;        high-angle elevation coverage in combination with an array providing lower elevation coverage;        progressive-phase-omnidirectional azimuth pattern;        reception of circularly polarized signals;        configurable with two arrays on one mast;        use of one common form of sub-array in dual arrays.        